This invention relates to antireflection coatings. More particularly this invention relates to a multilayer antireflection coating for use on transparent substrates, the coating having at least one pseudoinhomogeneous layer of which the index of refraction is disproportionally distributed in the direction of thickness.
Various types of antireflection coatings are now well known and are used on the surfaces of lenses, prisms and other transparent elements constituting part of optical instruments to reduce unwanted light reflection off the surfaces thereof and in turn to increase the transmittance.
One type of antireflection coatings is such that three layers of different materials having a high index of refraction and a low index of refraction are deposited on a transparent substrate one upon the other by any of the well-known techniques such as evaporation in a high vacuum. The antireflection coating has a design wavelength .lambda. in a spectral region wherein the reflectance does not exceed a certain specified value, and consists of a first layer having an optical thickness of one quarter of the design wavelength, a second layer having an optical thickness of one-half wavelength and a third layer having an optical wavelength of one quarter wavelength in the order in which they are deposited on the substrate. Each of the layers is a homogeneous layer of which the index of refraction is uniformly distributed throughout the layer.
In order for a three-layer antireflection coating of the type described as the basic form of coating design to reduce the surface reflectance to a lower value over a wider spectral region with maintenance of proper color balance that was previously possible with the three homogeneous layers, a solution has been proposed for the second layer. According to this proposal, the second homogeneous layer is replaced by an inhomogeneous layer of materials having a gently sloped distribution of the index of refraction in the direction of thickness, thereby to provide an improved performance three-layer antireflection coating. However, several difficulties are sometimes encountered in forming such an inhomogeneous layer, owing to the limitation of available materials for the layer, close relationship of the indices of refraction of the three layers relative to that of the substrate, and the requirement of much time and labor for carrying out the vacuum evaporation.